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How the radial body plan of echinoderms is related to the bilateral body plan of their deuterostome relatives, the hemichordates and the chordates, has been a long-standing problem. Now, using direct development in a sea urchin, I show that the first radially arranged structures, the five primary podia, form from a dorsal and a ventral hydrocoele at the oral end of the archenteron. There is a bilateral plane of symmetry through the podia, the mouth, the archenteron and the blastopore. This adult bilateral plane is thus homologous with the bilateral plane of bilateral metazoans and a relationship between the radial and bilateral body plans is identified. I conclude that echinoderms retain and use the bilateral patterning genes of the common deuterostome ancestor. Homologies with the early echinoderms of the Cambrian era and between the dorsal hydrocoele, the chordate notochord and the proboscis coelom of hemichordates become evident.

Origins of radial symmetry identified in an echinoderm during adult development and the inferred axes of ancestral bilateral symmetry